Production of conducting layers upon electrical insulating materials



Oct. 18, 1955 P. P. HOPF ET AL 2,721,154 PRODUCTION OF CONDUCTING LAYERS UPON ELECTRICAL INSULATING MATERIALS Filed June 25, 1950 onduct on [1/1 /2/1/ 1 I //7.5 u/aZ/hg Base Afiorneys United States Patent PRDDUCTION OF CGNDUCTING LAYERS UPON ELECTRICAL INEULATING MATERIALS Peter Paul Hopf, St. Pauls Cray, and Ronald Edgar John Lishmund, Reigate, England, assignors to Ward Blenkinsop & Company Limited, London, England, a British company Application June 23, 1950, Serial No. 169,854

Claims priority, application Great Britain June 24, 1949 7 Claims. (Ci. 117-226) This invention relates to the production of insulating materials, more particularly organic electrically insulating materials, coated with a surface layer, more especially an electrical conductor.

During the past few years an increasing demand for printed circuits has arisen as their potentialities have been more fully appreciated and the methods of producing them have improved. They are of especial value where compactness and light weight are desiderata. Components for such circuits such as coils can be produced in a similar manner to the circuits themselves. Some components require substantially uniform coatings over considerable areas and the methods hitherto available have not been such that a sufficient degree of uniformity has been available.

The coating of areas of insulating materials with conductors has been used as an intermediate step in the production of printed circuits. Such coating has been achieved for example by cathode sputtering in vacuum and by electrodeposition after applying a conducting layer. These two methods are difficult to carry out upon a large scale and are rather elaborate in the technique required and so expensive. A method is required which is more flexible, adapted to mass production and less expensive. From this point of view a more promising method is that in which a foil of metal is bonded to the base material by means of a bonding medium. However, it is diflicult to secure a high degree of uniformity by this method and the electrical and other physical properties of the bonding medium may be a limiting factor.

We have now found a process for coating the surface of electrically insulating materials in which a high degree of uniformity in the product is obtainable and which is simple to carry out. The process is applicable to other coating operations in which the materials are able to withstand a reasonable amount of heat and pressure without undergoing undesirable permanent change.

According to the present invention there is provided a process for producing an organic electrically insulating material coated with a layer of an electrical conductor which comprises applying a coating, free from a permanent binder, of a conductor in finely divided form to the surface of said material and applying heat and pressure to the coated surface, the temperature, pressure and time employed being suificient to cause the conductor to cohere together and to become at least partially embedded in the underlying material.

The starting materials employed in the present process are a finely divided form of the conductor such as finely divided silver, copper or carbon. This is made up into a slurry in a volatile diluent to which may be added a small proportion of a less volatile organic liquid which may act as a temporary adhering agent. Examples of suitable diluents are the lower alkanols, especially methanol and ethanol, the cyclic ethers such as dioxane, the lower aliphatic ketones, such as acetone and methyl ethyl ketone, and water. In selecting the diluent regard must be had for any undesirable action, such as a swelling "ice action, which it might have upon the surface of the insulating material. Typical examples of conductors useful in the process are finely divided silver obtained by filtering and washing the composition prepared according to the copending application Serial N0. 777,116 filed September 30, 1947, and now Patent 2,592,870, finely divided copper, carbon black and colloidal graphite. Examples of less volatile organic liquids which may be present in the slurries are the lower polyhydric alcohols, such as ethylene glycol and glycerine and the aminoalcohols, such as monoethanolamine. The proportion by weight of solids in the slurry varies greatly depending upon the material. In the case of finely divided silver the proportion may be about 4 to 1 Whilst with carbon black 1 to 1 is suitable.

The organic electrically insulating material may be a thermoplastic or thermoset material or it may be a high polymer. It may be loaded with inorganic insulating materials such as asbestos, mica or glass fibres. EX- amples of suitable materials are electrical grades of phenol-formaldehyde and aniline-formaldehyde synthetic resins, amino-plast resins, such as urea-formaldehyde and melamine-formaldehyde resins, polyacrylic resins, such as polymethyl methacrylate, polyvinyl resins, such as polystyrene, and polythene. There may also be used mixtures of thermoplastic and thermoset resins such as the so-called styrene copolymers and the products obtained by the use of such mixtures as phenol-formaldehyde condensation products with a minor proportion of polyvinyl chloride. These materials may be used in a variety of forms such as sheet laminates, wound laminate forms such as circular, elliptical and polygonal, are shaped laminates, extruded forms, films, and filled plates and blocks.

In describing one form of the invention it will be assumed that it is desired to produce a coating of silver upon a sheet of polymethyl methacrylate. The sheet material is first substantially uniformly coated with finely divided silver. For this purpose a slurry of sponge silver in methanol may be sprayed on to the surface of the polymethylmethacrylate sheet. The greater part of the methanol is then removed by drying: a small amount is retained for adhering the silver to the sheet. The sheet is then passed between a pair of rollers, the one in contact with the coated surface being heated. The pressure at the nip of the rollers should be adjustable to any desired value. It is preferred to apply the maximum possible pressure and the lowest possible temperature in this operation. If desired the coated surface can be preheated, as by radiant heat, immediately prior to passing to the rollers but if this is done it may be necessary to displace the atmosphere in contact with the coated surface so as to prevent oxidation: nitrogen is suitable for this purpose. In the case of polymethylmethacrylate a temperature of about C., a pressure of about 500 lbs/in. and a time of contact of about 3 seconds is suitable. Coating, drying and pressing may be carried out as a continuous operation. The product has, in cross section, the appearance shown in the single figure of the drawings, the conducting layer consisting of highly compressed metallic silver particles, the lower layers of which are embedded in the underlying insulating polymethyl methacrylate.

It is frequently more convenient to use a press having a heated swinging platen and to use the sheet material in pieces which have been cut to a size suitable for the press. After a piece of the coated polymethyl methacrylate has been mounted in the press the smooth surfaced and heated platen is swung into contact with the coated surface of the material and held in contact for a predetermined time. The platen is then swung back and the sheet removed from the press. In this case the tem- 7 short times of contact are involved.

on both sides.

. Without a permanent binder being present.

perature may be 145 C., the pressure 500 lbs/in. and the time of contact 6 seconds. In this Way a succession of like dimensioned sheets can be treated in quite a short time. a 7

When materi ls having a :fairly well defined melting point are treated, the pressing operation may be carried out with a heated roller or platen at a emperature somewhat above the melting point of the material providing the time of pressing is short. Thus polythene can be pressed at a temperature some C. above its melting point provided that the time of contact is sufliciently shor g. :1 s cond. with thermo tting mate i s qui e high temperatures, such as 200250 .C., and high pressures, such as 1000-2000 lbs/in. are preferred. With this latter type ofmaterial the whole of the surface need not have been coated .e, g. a. margin may have been left Materials which soften on heating may be printed using a roller or platen surface maintained at a very mu h higher t mp ra e th n th at which th y first soften. To some extent the time of contact appears to depend upon the area when a press is used.

Passage between two rollers is best used only when It will be appreciated that more than one pair of rollcrs can be used in eri s a hat different temp ratur s an pre s s can be used at succeeding stages. V 7

An important feature of the process is that the conductor is applied to the surface .of the insulating material A small amount of higher boiling organic liquid or the residue Of the volatile diluent is relied upon to maintain a sufii cient adherence of the particles of conductor to the insulating material and this is volatilised when heat and pressure is applied. In the product the particles firmly cohere to each other and at least the lower layers thereof are embedded in the insulating material.

The higher boiling organic liquid may be so chosen as to counteract any tendency of the conductor to undergo chemical change in the earlier stages of the process. Thus when sponge silver is used a small addition of an ethanola-mine may be made to prevent oxidation to silver oxide or to reverse such change, if it has occurred, during the hot pressing. However, since the reduction involved is an exothermic process it is not possible to use silver oxide per se in the coating step.

The product obtained may be cut into suitable sized and shaped portionsaccording to the purpose for which it is intended e. g. for internal aerials for radio and television receivers, for plates for stacked capacitors, strips for winding for wound capacitors and electricfshields when a conducting metal such as silver or copper or a mixture thereof, is used or for resistances when Carbon 7 .black or graphite is used.

It will be appreciated that the Present technique be used to produce coatings of different thicknesses and that more than one coating can be applied to the same side of a material. It will also be understood that both sides of sheet material laminates etc. can be coated with the same or with difierent coating materials: it is preferred however to coat one at a time;

The present invention can be used in conjunction with that described in our application Serial No. 164,926, May 29, 1950, wher in e have described h p duction-of printed circuits by a technique similar to that herein disclosed. Thus, a printed circuit may be pr duced as therein described upon one side of .an organic insulating material laminate and an electric shield for that circuit may be produced upon the other side of the laminate in accordance with the present invention.

The following examples illustrate the manner in which the invention may be carried into effect:

Example 1 A Poly ene s ee ha ng a melt ng point of abou 1.15 and 0.03 inch thicknes as empl yed- I wa 0.0625 inch thick was employed,

0.125 inch thick was employed.

sprayed with a slurry of sponge silver in methanol containing 2% of g'lycerine. The slurry contained parts by weight of silver for each 20 parts of diluent by weight. Air drying was used to evaporate the greater part of the diluent. a pair of nip o the uppe f hich was run at approximately C. The pressure at the nip was 50 lbs/in. and the sheet was run through at such a speed that t e ime of con ac did no exc e ne e d- Example 2 Example 3 A polystyrene film, 0.0005 inch thick was sprayed with sponge silver having an average particle size of 50 as described in Example 1. It was passed between a pair of rollers adjusted to a pressure of 20 lbs/in. at the nip.,

Temperatures between 125 and were employed and the rate of passage was 4 feet per minute.

Exampl 4 An electrical grade phenol-formaldehyde laminate It was sprayed with sponge silver using methanol containing 5% of ethanolamine by weight, the slurry containing 80 parts of silver for each 20 parts of diluent by weight. jThe sprayed surface was substantially dried using infra-red heat and it was then transferred to a press the swinging platen of which was run at 200 C and a pressure of l250 lbs./in. the time of contact was 4 seconds.

Example 5 1 An electrical grade of filled aniline-formaldehyde sheet material was sprayed with electrolytic copper powder 400 mesh using methanol containing 5% .of ethanolamine by weight, the slurry containing 80 parts of copper for each 20 parts of diluent by weight. The sprayed surface was substantially dried using infra-red heat and it was then transferred to a press and pressed for 4 seconds at 9 C. using 1,000 lbs/in. pressure.

Exa p e A urea-formaldehyde laminated sheet, paper laminated, It was sprayed with a mixture .of 60% by weight of electrolytic copper powder .(400 mesh) and 4.0% by weight of sponge silver in a dispersion containing 8.0% by weight'of the metals, 15% by weight .of ethanolamine .and 5% by weight of methanol. It was substantially dried using infra-red heat and transferred to a press the swinging platen of which was run at 180 .C. and a pressure of 1,000 lbs/in} was applied for 1.0 seconds.

What we claim is:

1. A process for producing an organic electrically insulating material coated with .a layer of an electrical nductor which comprises applying to the smooth surface of a synthetic electrically insulating organic plastic material having at least one smooth surface a slurry .of a nnely divided electrical conductor in a volatile inert liq i said s u ry b ng bs antially fr e t am non: latile organic ma ials a d s d o at e iqu d heirs wi h ut u s t al sw lling ct n upon sa plastic material, evaporating the greater part .of said yolatile liquid and, P QI o c mpletion of evaporation hereof and while there is still sufiicient liquid to adhere said The substantially dried sheet was run through electrical conductor to said plastic material, simultaneously applying heat and pressure to the coated surface to complete evaporation of said volatile liquid and to cause the individual particles of conducting material to cohere and to become at least partially embedded in the underlying insulating material and without substantially affecting the electrical properties of the surface of said material.

2. A process for producing an organic electrically insulating material coated with a layer of an electrical conductor which comprises applying to the smooth surface of a synthetic electrically insulating organic plastic material having at least one smooth surface, a slurry of a finely divided electrical conductor in a mixture of at least two volatile inert miscible liquids, one of said liquids being more volatile than the other, said slurry being substantially free from non-volatile organic materials and said volatile liquids being substantially without swelling action upon said plastic material, evaporating the greater part of said volatile liquids, and, prior to completion of evaporation of the less volatile of said volatile liquids and while there is still sufficient liquid to adhere said electrical conductor to said plastic material, simultaneously applying heat and pressure to the coated surface to complete evaporation of said volatile liquid and to cause the individual particles of conducting material to cohere and to become at least partially embedded in the underlying insulating material and without substantially affecting the electrical properties of the surface of said material.

3. A process for producing an organic electrically insulating material having at least one surface coated with an electrically conducting metal coating which comprises applying to the smooth surface of a synthetic organic electrically insulating plastic material having at least one smooth surface a slurry of a finely divided electrical conducting metal in a volatile inert liquid, said slurry being substantially free from any non-volatile organic materials and said volatile liquid being without substantial swelling action upon said plastic material, evaporating the greater part of said volatile liquid and, prior to completion of evaporation thereof and while there is still suflicient liquid to adhere said electrical conductor to said plastic material, simultaneously applying heat and pressure to the coated surface to complete evaporation of said volatile liquid and to cause the individual particles of conducting material to cohere and to become at least partially embedded in the underlying insulating material and without substantially affecting the electrical properties of the surface of said material.

4. A process for producing an organic electrically insulating material having at least one surface coated with silver which comprises applying to the smooth surface of a synthetic organic electrically insulating plastic material having at least one smooth surface a slurry of finely divided metallic silver in a volatile inert liquid, said slurry being substantially free from any non-volatile organic materials and said volatile liquid being without substantial swelling action upon said plastic material, evaporating the greater part of said volatile liquid and, prior to completion of evaporation thereof and while there is still sutficient liquid to adhere said metallic silver to said plastic material, simultaneously applying heat and pressure to the silver coated surface to complete evaporation of said volatile liquid and to cause the individual particles of metallic silver to cohere and to become at least partially embedded in the underlying insulating material and without substantially affecting the electrical properties of the surface of said material.

5. A process for producing an organic electrically insulating material having at least one surface coated with silver which comprises applying to the smooth surface of a synthetic organic electrically insulating plastic material having at least one smooth surface a slurry of finely divided metallic silver in a volatile inert liquid containing a minor proportion of a volatile organic reducing agent miscible therewith and capable of reducing silver oxide to silver at raised temperatures, said slurry being substantially free from any non-volatile organic materials and said volatile liquid and said reducing agent being without substantial swelling action upon said plastic material, evaporating the greater part of said volatile liquid, and, prior to completion of evaporation thereof and of said volatile organic reducing agent and while there is still sufficient liquid to adhere said metallic silver to said plastic material, simultaneously applying heat and pressure to the silver coated surface to complete evaporation of said volatile liquid and said reducing agent and to cause the individual particles of metallic silver to cohere and to become at least partially embedded in the underlying insulating material and without substantially afiecting the electrical properties of the surface of said material.

6. A process for producing an organic electrically insulating material having at least one surface coated with carbon black which comprises applying to the smooth surface of a synthetic organic electrically insulating plastic material having at least one smooth surface a slurry of finely divided carbon black in a volatile inert liquid, said slurry being substantially free from any non-volatile organic materials and said volatile liquid being without substantial swelling action upon said plastic material, evaporating the greater part of said volatile liquid and, prior to completion of evaporation thereof and while there is still suflicient liquid to adhere said carbon black to said plastic material, simultaneously applying heat and pressure to the coated surface to complete evaporation of said volatile liquid and to cause the individual particles of carbon black to cohere and to become at least partially embedded in the underlying insulating material and Without substantially afiecting the electrical properties of the surface of said material.

7. A process for producing an organic electrically insulating material having at least one surface coated with graphite which comprises applying to the smooth surface of a synthetic organic electrically insulating plastic material having at least one smooth surface a slurry of finely divided graphite in a volatile inert liquid, said slurry being substantially free from any non-volatile organic materials and said volatile liquid being without substantial swelling action upon said plastic material, evaporating the greater part of said volatile liquid and, prior to completion of evaporation thereof and while there is still sufficient liquid to adhere said graphite to said plastic material, simultaneously applying heat and pressure to the coated surface to complete evaporation of said volatile liquid and to cause the individual particles of graphite to cohere and to become at least partially embedded in the underlying insulating material and without substantially affecting the electrical properties of the surface of said material.

References Cited in the tile of this patent UNITED STATES PATENTS 1,149,974 Chisholm Aug. 10, 1915 1,922,254 McCulloch Aug. 15, 1933 1,987,969 Parkin Jan. 15, 1935 2,034,008 Taylor Mar. 17, 1936 2,060,114 Podolsky Nov. 10, 1936 2,136,370 Bockius et al Nov. 15, 1938 2,177,484 Fruth Oct. 24, 1939 2,426,609 Hodgdon Sept. 2, 1947 2,492,429 John Dec. 27, 1948 OTHER REFERENCES Synthetic Organic Chemicals, 12th ed., July 1, 1946, pp. 77, 78 and 79.

Printed Circuit, National Bureau of Standards Circular 468, November 15, 1947, pp. 7 and 28. 

1. A PROCESS FOR PRODUCING AN ORGANIC ELECTRICALLY INSULATING MATERIAL COATED WITH A LAYER OF AN ELECTRICAL CONDUCTOR WHICH COMPRISES APPLYING TO THE SMOOTH SURFACE OF A SYNTHETIC ELECTRICALLY INSULATING ORGANIC PLASTIC MATERIAL HAVING AT LEAST ONE SMOOTH SURFACE A SLURRY OF A FINELY DIVIDED ELECTRICAL CONDUCTOR IN A VOLATILE INERT LIQUID, SAID SLURRY BEING SUBSTANTIALLY FREE FROM NONVOLATILE ORGANIC MATERIALS AND SAID VOLATILE LIQUID BEING WITHOUT SUBSTANTIAL SWELLING ACTION UPON SAID PLASTIC MATERIAL, EVAPORATING THE GREATER PART OF SAID VALATILE LIQUID AND, PRIOR TO COMPLETION OF EVAPORATION THEREOF AND WHILE THERE IS STILL SUFFICIENT LIQUID TO ADHERE SAID ELECTRICAL CONDUCTOR TO SAID PLASTIC MATERIAL, SIMULTANEOUSLY APPLYING HEAT AND PRESSURE TO THE COATED SURFACE TO COMPLETE EVAPORATION OF SAID VOLATILE LIQUID AND TO CAUSE THE INDIVIDUAL PARTICLES OF CONDUCTING MATERIAL TO COHERENT AND TO BECOME AT LEAST PARTIALLY EMBEDDED IN THE UNDERLYING INSULATING MATERIAL AND WITHOUT SUBSTANTIALLY EFFECTING THE ELECTRICAL PROPERTIES OF THE SURFACE OF SAID MATERIAL. 